Silicon wafers or sheets may be used in, for example, the integrated circuit or solar cell industry. Demand for solar cells continues to increase as the demand for renewable energy sources increases. One major cost in the solar cell industry is the wafer or sheet used to make solar cells. Reductions in cost to the wafers or sheets may reduce the cost of solar cells and make this renewable energy technology more prevalent. One promising method that has been investigated to lower the cost of materials for solar cells is the horizontal ribbon growth (HRG) technique where crystalline sheets are pulled horizontally along the surface of a melt. In this method, a portion of a melt surface is cooled sufficiently to locally initiate crystallization with the aid of a seed, which may be then drawn along the melt surface to form a crystalline sheet. The local cooling may be accomplished by providing a device that rapidly removes heat above the region of the melt surface where crystallization is initiated. Under proper conditions a stable leading edge of the crystalline sheet may be established in this region.
In order to ensure growth stability, it may be useful to control heat flow through the melt in the region of the leading edge of the crystalline sheet that is drawn from the melt. However, achieving controlled heat flow within a silicon melt is very challenging for several reasons. Firstly, molten Si has a very high thermal conductivity, so that any heat introduced at the bottom of a crucible that contains the melt spreads out before reaching the melt surface. In addition, fused silica is often used as the crucible material used to contain silicon melt, due to its resistance to reaction with silicon at elevated temperature. However, fused silica is a good thermal insulator which generates a large thermal gradient when substantial heat flow is conducted from outside the crucible into the silicon melt. In turn, this that the outer temperature of the fused silica crucible being heated is maintained at a much higher temperature than the melt temperature. Additionally, a temperature drop is incurred when heat flows from a bottom of a crucible to the surface of a melt, which is proportional to the melt depth of the melt within a crucible. In the case of horizontal growth of crystalline sheets from a silicon melt a melt thickness in the range of 10-15 mm is commonly employed to achieve stable processing conditions. However, fused silica softens to an unacceptable degree above 1880 K, which limits the amount of heat flow that can be introduced by a heat source into the bottom of a crucible while still maintaining a needed melt temperature at the surface of the melt. This limited heat flow provided by present day apparatus may not be sufficient to ensure desired quality of a crystalline sheet grown from the melt surface.
It is with respect to these and other considerations that the present improvements have been needed.